When a patient has a pacemaker, the electrical pulses it provides are added to the EKG waves derived from that patient by the electrodes of an electrocardiograph. Depending on the pacemaker employed and its adjustment, the duration of the pace pulses may be from about 100 to about 2000 microseconds, and their amplitude at most electrode locations usually exceeds that of the EKG wave by a substantial amount. Pacemakers in which the pace pulses are capacitively coupled to the patient cause a voltage of the opposite polarity that can last for as long as several milliseconds following the pace pulse. Hereinafter, this voltage will be referred to as the after-voltage.
A conventional electrocardiograph displays a single EKG wave, but electrocardiographs have recently become available that display a plurality of EKG waves at the same time by rapidly sampling each wave in sequence and using the samples derived from each wave to reproduce it. Thus, the voltages derived from different electrodes on the patient's body can be simultaneously displayed. Due to limitations in slew rate and frequency response, conventional electrocardiographs may be disturbed by the high amplitude of a pace pulse so as to display it with an incorrect amplitude, and both types of electrocardiographs can respond to the after-voltage so as to display a pulse of the wrong polarity. Electrocardiographs utilizing sampling techniques display representations of the pace pulses that vary in amplitude depending on the phase relationship between the sampling times and the pace pulses, and no pace pulses will be displayed if no portion of a pace pulse occurs during a sample.